This invention relates to screw threaded fastening devices which are designed for removal only by authorized persons. The invention is particularly useful as a lug fastener for an automobile wheel to prevent unauthorized removal of the fastener and theft of the wheel.
Tamper-proof threaded fastenings are very much in demand, especially for automobile wheels. The so-called "mag" wheels for sport cars are especially vulnerable to theft, and are very costly, and therefore attractive to thieves.
A number of products are now offered to "lock" automobile wheels to the wheel hubs. One of the most popular designs for these wheel locks is offered by an organization called "McGard" of Buffalo, N.Y. The McGard locking devices consist of a set of four special lug nuts or lug bolts together with a special "key" wrench. One special lug nut or bolt is used for each wheel of an automobile. Each lug nut or lug bolt has an enlarged head with a shaped annular channel in the outer axial end of the head designed to receive the special key wrench. The annular channel has an irregular shape, and the key wrench has a corresponding matched irregular annular shape. The annular channel in the head is fairly shallow, and the channel is of substantially uniform width, which is apparently cut out by a routing tool. This device has a number of very important disadvantages and problems as follows:
The enlarged head of the fastener has a substantially greater weight than the conventional lug fasteners used on the same wheel. This tends to unbalance the wheel for high speed operation. That is a substantial disadvantage for a high performance automobile. The McGard organization recognizes this disadvantage because it recommends that the wheels be balanced with the fasteners in place on each wheel, and that the locking fastener then be carefully mounted on the wheel in the exact position where the locking fastener appeared at the time of balancing.
Another important disadvantage is that the McGard locking fastener has a substantially different appearance from the other lug fasteners for the wheel. This is a very important disadvantage to the automobile owner who has often made a considerable investment in special wheels to achieve the ultimate in appearance for his automobile.
Another disadvantage emphasized by McGard is that power torque wrenches must not be used for tightening the McGard locking fastener on the wheel without damaging the locking fastener, or to the key wrench, or both. A corollary of this disadvantage is that the locking fastener simply cannot be tightened as much as the other convenient lug fasteners on the wheel. This means that the locking fastener does not do its proportionate share of holding the wheel on the hub of the automobile, and may be more easily subject to unintentional loosening. If the locking fastener comes loose, it cannot perform its function as a lock.
Still another important disadvantage of the prior locking fastener is the extreme difficulty in engaging the key wrench into the annular channel in the locking fastener. This is an important disadvantage, particularly if tires must be changed quickly on the road in the dark, or under adverse conditions.
Accordingly, the objects of the present invention include separately, and collectively, overcoming or minimizing each of the above mentioned disadvantages and problems encountered with the prior art locking devices.
Further objectives and advantages of the invention will be apparent from the following description and the accompanying drawings.